A New GRB Detection Era
Sadly, on June 4, 2000, after 9 years of service, the Compton gamma-ray observatory satellite
was de-orbited due to the failure of one of its gyros. This was asomewhat controversial
decision, as the CGRO instruments covered an unprecedented six decades of the
electromagnetic spectrum, from 30 keV to 30 GeV, andits BATSE detector had been the major
GRB event-detection system. BeppoSax wasstill in orbit, but its main role had been X-ray
astronomy and pinpointing GRBpositions. Thus, something of a lull occurred in GRB alerts
until the next generation of satellites were launched, namely HETE-2 in October 2000, and
thenINTEGRAL (2002), and SWIFT (2004). SWIFT in particular is a formidable toolfor
detecting GRB outbursts and analyzing the decay of the glow. Its primaryobjective is to use its
BAT (Burst Alert Telescope) to discover a new GRB and relayits position (typically with a
positional accuracy of between 1 and 4 arcminutes) tothe ground while simultaneously slewing
itself to bring the GRB within the field ofits X-ray (XRT) and ultraviolet-optical (UVOT)
telescopes. However, history hasshown that science always progresses fastest when as much
data, from as manysources as possible, is collated. In addition, satellites have a limited lifetime,
andprofessionals greatly value accurate amateur observations, so the amateur still hasa role to
play. Above and beyond these points there is still tremendous satisfactionto be had from
imaging the dying glow of a colossal explosion at the edge of theobservable universe from your
own backyard.
GRB 050509B
On May 9, 2005, a short (sub 2-second) GRB was detected that, for the firsttime, enabled the
pinpoint precision in space of this kind of short-durationburst. No optical transient was detected
for this GRB, and therewas no evidence of any supernova being involved. This considerably
strengthened the argument that short-duration GRBs were caused by a different mechanism
to the long-duration supernova-based GRBs. See the earlier short-durationGRB section for
more details. Assuming the source was located within theelliptical galaxy its redshift of z =
0.226 places the GRB roughly 2.7 billionlight-years from Earth.
GRB 050724
The July 24 2005 GRB is still one of the most intensively studied short-duration
GRBs to date. The Swift X-ray telescope, NASA’s Chandra telescope, and the giant
Keck telescope in Hawaii all focused on the spot where the GRB had erupted.
Although the GRB itself was of short duration the X-ray afterglow had a long,
flaring characteristic. Professional astronomers have
speculated that a black hole swallowing a neutron star was a possible cause of this
GRB. The host galaxy’s redshift of z = 0.258 implies a distance from Earth of
roughly 3 billion light-years.
GRB 050904
At the time of writing the GRB detected at 01:51:44 UT on September 4, 2005, is
still the most remote GRB yet detected. As with the October GRB 021004, discovered 3 years
earlier, it was located in Pisces, but unlike that GRB its light was so
seriously redshifted there was no hope of amateur astronomers imaging the optical
afterglow. relationship between
observer and source time durations caused by the time dilation effect, and this
GRB had a colossal redshift of z = 6.3! This means that the GRBs light has traveled
for 13 billion years to get to us, and it was emitted when the universe was far less
than a billion years old. In addition, the actual burst duration was probably only
about 30 seconds. Once again, the progenitor was assumed to be a massive supernova
collapsing into a black hole.
GRB 060218 = SN 2006aj
GRB 060218 occurred in the constellation of Aries, not far from the border with
Taurus. The GRB had some unusual properties, including a very long burst, and its
near simultaneous appearance with the supernova 2006aj indicated this was, once
again, the result of a giant star collapsing into a black hole as a Type Ib/Ic supernova. For a
collapsar hypernova in a
galaxy as close (a relative term!) as 440 million light-years, a peak magnitude of
17.4 was not exceptional. Nevertheless, it was yet another link between longduration GRBs
and supernovae.
GRB 060505
GRB 060505 presented astronomers with another puzzle. Here was a long-
(4-second) duration GRB associated with a galaxy that was not at the edge of the
observable universe (z = 0.089, distance = 1.2 billion light-years), and yet there was
no evidence for any associated supernova, even when examined with the Hubble
Space Telescope. The faint afterglow was traced to a prominent star-forming
region in the Sc-type galaxy with the rather technical name 2dFGRS S173Z112. Either the
physical mechanism was the same as that for short-duration GRBs (i.e.,
a double neutron star or neutron star/black hole merger), but for some reason the
resultant GRB had a very long duration, or, it was a standard collapsar but with no
visible supernova. The latter explanation could be the case if the star collapsed so
quickly into a black hole that there was no time for a traditional luminous outburst.
Astronomers still disagree over this one, but the evidence perhaps favors the
collapsar/black hole theory, as most of the evidence, except the lack of an optical
supernova, is the same as for the other collapsar/long GRB examples.
How to Observe GRBs
In the final years of the twentieth century (which was not all that long ago)
detecting the optical counterpart of a fading GRB was an event that carried a lot
of kudos, in both the amateur and professional communities. Amateurs were well
placed, with their CCD equipment, to react quickly and image objects well below
20th magnitude. In addition there were, initially, no X-ray/gamma-ray satellites
that could pinpoint a high-energy transient to better than 10 arcminute precision. They were
scattered around the globe at
many latitudes and longitudes, so not all of them could be clouded out at the same
time, and some of those with clear skies would always be on the night-time side of
Earth experiencing darkness and with the GRB field above their horizon. In
addition, in the early days, before the importance of GRBs was fully appreciated,
it could be quite difficult to interrupt an observing run on a world class telescope,
which was already fully oversubscribed by professional astronomers with their
own agendas. In addition,
the speed with which ground-based astronomers are alerted to a GRB is very
rapid now, and professional astronomers at many institutions are well aware
that a bright, well-placed GRB overrides all other observing programs. The
modest (by professional standards) 2-m Faulkes telescopes can respond very
quickly to GRB alerts and have far more light grasp than even the largest
amateur telescopes.
Still, the speed with which amateurs are alerted is rapid, too, and although the
field of view issue is no longer a consideration, amateurs still have advantages
where the optical transients of some GRBs are concerned. In addition, a far
southern hemisphere facility cannot image the far northern hemisphere of the
sky and vice versa.
Where a bright supernova is associated with a GRB the amateur can play a vital
role monitoring the decline of the object over weeks and months, a time allocation
few professional observatories can justify. GRB 030329A
(described later), and a few more amateurs have witnessed the more steady light
of an associated supernova from a GRB, but actually witnessing a bright GRB
optical transient fading (and maybe flickering) with your own eye must be a goal
that many amateurs would strive for.
To be fully prepared for the next GRB event you need to subscribe to some form
of alert system. All astronomers (that I know) are on e-mail and most have cell
(mobile) phones, with a few having electronic pagers, too. By giving your contact
details to an organization issuing alerts, such as the AAVSO or, in Britain, The
Astronomer magazine, you can sign up to the most convenient system. Of course, when you
slew to the region of a new GRB the CCD field will be totally
unfamiliar, just a sea of stars, anyone of which might be the optical component of
the GRB. If you are lucky someone might produce a finder chart and put a link to it
in the AAVSO high energy e-mail alerts. However, more likely is that you will need
to visit the Space Telescope Science Institute’s (STScI) Digitized Sky Survey (DSS)
pages at http://archive.stsci.edu/cgi-bin/dss form and download a chart of the
field, to look for a new faint object in the field of view.
